I own a 98 f-150 5.4 with 70,000 miles.

I get a check engine light (code p0455-large leak detected in evaporative emission control system).

replaced the gas cap with an after market 'stant' ( I believe was the manufacturer) brand. The check engine light went away for 4 days then came on again.

no obvious vacuum line leaks.

puzzled. any suggestions?